[Minor][Dev] Wiki errors

[VincentFP]

In short, this page is misleading and would need a few updates :)

Issue 1 - webpack.config.js > webpack.config.mjs

Copy over the webpack.config.js file from packages/core and change things accordingly:
should be changed to:
... webpack.config.mjs ...

to avoid

Cannot use import statement outside a module

Issues 2 - karma.conf.js updated not reflected in the docs

The provided snippet in the wiki should be updated in favor of:

/*
 * Copyright 2022 Palantir Technologies, Inc. All rights reserved.
 */

module.exports = async function (config) {
    const { createKarmaConfig } = await import("@blueprintjs/karma-build-scripts");
    config.set(
        createKarmaConfig({
            dirname: __dirname,
            coverageExcludes: [
            ],
        }),
    );
};

to avoid

Error in config file!
  Error [ERR_REQUIRE_ESM]: require() of ES Module /blueprint/packages/karma-build-scripts/index.mjs not supported.
Instead change the require of /blueprint/packages/karma-build-scripts/index.mjs to a dynamic import() which is available in all CommonJS modules.